<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        div.parent{
            width:200px;
            height:200px;
            background-color:aqua;
        }
        div.child{
            width:200px;
            height:200px;
            background-color:red;
            opacity: 0.5;
        }
    </style>
</head>
<body>
    <div>
        <div class="parent">
            <div class="child"></div>
        </div>
    </div>
</body>
</html>